Research ArticleELECTRICAL CONDUCTIVITY

Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges

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Science Advances  06 May 2016:
Vol. 2, no. 5, e1501631
DOI: 10.1126/sciadv.1501631
  • Fig. 1 Plot of the logarithm of EC of chlorite as a function of reciprocal temperature.

    The inverted triangles and circles refer to data at 2 and 4 GPa, respectively. The first stage of the heating cycle indicates low electrical conductivities (red inverted triangles and blue circles). Upon heating to ~923 K at 4 GPa, that is, beyond the thermodynamic stability field of chlorite (Chl), a discontinuous enhancement of EC by more than one order of magnitude is observed (blue filled circles). A subsequent enhancement of EC by more than two orders of magnitude (gray circles) is also observed after the sample is kept at 923 K. Activation enthalpies are reported in eV, next to the individual fits. The uncertainties in the estimation of the EC result from the estimations of temperature, pressure, sample dimensions, and data-fitting errors and are less than 5%. The green dashed vertical line indicates the thermal stability of chlorite. Mgt, magnetite; Ol, olivine; Prp, pyrope.

  • Fig. 2 Electron backscattered images of the recovered samples after the EC measurements.

    (A) Recomposed colored map of the chemical composition of the recovered sample after dehydration at 4 GPa and 973 K. The presence of aqueous fluid in the sample at high pressure and temperature is suggested by the presence of voids (darker regions). The magnetite grains along relict chlorite foliation planes and fracture lines are denoted by white and purple. (B) The close-up view of the mineral assemblage of olivine, pyrope, and magnetite is formed by dehydration of chlorite. (C) Backscattered electron image of the chlorite sample before the experiments.

  • Fig. 3 Magnetotelluric data reported for a variety of subduction zones (pink vertical lines) compared with the range of possible values of EC related to the chlorite dehydration.

    The shaded purple, blue, and pink regions indicate EC dominated by hydrous minerals, aqueous fluid, and interconnected magnetite, respectively. The conductivity of talc (Tlc) (14, 15), serpentine (Serp) (11), dehydrating fluids (14, 15), and brine with 3 and 10 wt % of NaCl (17) is also shown for comparison. The red line in the embedded figure (in the upper right corner) indicates the EC of magnetite observed in our sample as a function of magnetite volume fractions (see fig. S1 for details of the calculations). The blue dashed lines indicate the volume fractions of magnetite required to explain the EC of various subduction systems. The magnetotelluric models presented in the figure are Bolivia (BL) (5), Cascadia–British Columbia (C-BC) (2), Chile (CL) (6), Cascadia-Oregon (C-O) (8), Costa Rica (CR) (10), Kyushu (KY) (1), Mariana (MR) (9), and New Zealand (NZ) (7).

  • Fig. 4 Geodynamical model explaining the origin of the high-conductivity region in the shallow mantle wedge.

    First, dehydration of hydrous minerals such as serpentine in the subducting slab generates aqueous fluid. The aqueous fluid leads to rehydration of the mantle wedge and formation of a hydrous phase such as serpentine and chlorite. These mineral phases in the mantle wedge are dragged by corner flows, where chlorite undergoes dehydration when it crosses the wedge mantle isotherm at about 923 K. The dehydration of chlorite leads to the formation of an interconnected network of magnetite grains, which, in turn, explains the very high conductivity measured geophysically. Finally, the dehydration produces a secondary fluid, inducing mantle melting, which creates arc volcanoes, with mantle sources located at a fixed depth of 120 ± 40 km (36).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/5/e1501631/DC1

    fig. S1. EC as a function of volume fraction of magnetite in chlorites.

    fig. S2. Electron backscattered images of the nickel (Ni) electrodes after the EC measurements.

    fig. S3. Impedance spectra of the sample at different stages of heating.

    table S1. The chemical composition of chlorite and its dehydration products.

    table S2. The fitting parameters for chlorite, aqueous fluid, and magnetite.

    References (38, 39)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. EC as a function of volume fraction of magnetite in chlorites.
    • fig. S2. Electron backscattered images of the nickel (Ni) electrodes after the EC measurements.
    • fig. S3. Impedance spectra of the sample at different stages of heating.
    • table S1. The chemical composition of chlorite and its dehydration products.
    • table S2. The fitting parameters for chlorite, aqueous fluid, and magnetite.
    • References (38, 39)

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